1. Field of the Invention
The present invention relates generally to semiconductor devices having multi-layer film structures, and more particularly to an improved semiconductor device of a multi-layer film structure in which a lower conductor layer and an upper conductor layer separated from each other by a multilayered insulator film are electrically connected to each other in a satisfactory state through a contact hole provided in the multilayered insulator film. This invention further relates to a method of manufacturing such a semiconductor device having a multi-layer film structure.
2. Description of the Background Art
Recently, high integration of semiconductor devices is remarkable, and a technique of forming a contact hole in a multi-layer film is required for such high integration.
FIG. 3A is a plan view of a conventional dynamic random access memory (DRAM) having a multi-layer film; and FIG. 3B is a cross sectional view taken along the lines IIIB--IIIB of FIG. 3A. Referring to these figures, the DRAM includes a semiconductor substrate 1 of the P-type. An isolating oxide film 2 for isolating device areas from each other is formed on a main surface of the semiconductor substrate 1. The DRAM shown in FIG. 3 basically includes transistors and stacked capacitors. Each of the transistors includes an N-type source/drain region 3 formed in the main surface of the semiconductor substrate 1, a transfer gate oxide film provided on the substrate 1, and a transfer gate 5. Meanwhile, each of the stacked capacitor includes a storage node 6, a capacitor dielectric film 7 provided on the storage node 6, and a cell plate electrode 8 provided on the capacitor dielectric film 7. The storage node 6 and the transfer gate 5 are isolated from each other by a first layer insulating film 9 between them. The layer insulating film 9 is formed of a silicon oxide film while the capacitor dielectric film 7 is formed, for example, of a silicon nitride film.
A second layer insulating film 10 is provided over an entire surface of the semiconductor substrate 1 including the transistors and stacked capacitors. The second layer insulating film 10 has a contact hole 11 formed therein which extends to the source/drain region 3. A bit line 12 extends through the contact hole 11 and is connected to the source/drain region 3. A third layer insulating film 13 is provided on the bit line 12. An aluminum wire 14 for carrying a signal to the transfer gate 5 is formed on the third layer insulating film 13. The aluminum wire 14 is made of an aluminum-silicon-copper alloy and formed by sputtering.
With the DRAM having such a construction as described above, when word lines are selected and a predetermined potential is applied to the transfer gate 5, the source/drain regions 3, 3 are rendered conductive to perform a reading/writing operation.
Subsequently, a structure of a contact portion between an aluminum wire 14 and a transfer gate 5 will be described.
FIG. 4 is a cross sectional view of a conventional structure of the contact portion between the aluminum wire and transfer gate, taken along the lines IV--IV of FIG. 3A.
Referring to FIG. 4, a multi-layer film 15 consisting of a first layer insulating film 9, an extension 7a of a capacitor dielectric film 7 and a second layer insulating film 10 is formed on a transfer gate 5. A contact hole 16 is formed in the multi-layer film 15, an aluminum wire 14 extends through the contact hole 16 and is connected to the transfer gate 5. The aluminum wire 14 is formed by sputtering as described hereinabove.
Now, referring to FIGS. 3A and 3B when the bit line 12 is to be connected to the source/drain region 3 by way of the contact hole 11, polycrystal silicon is placed into the contact hole 11 by chemical vapor deposition. Since such a chemical vapor deposition has a good coating property with respect to a sidewall face of the contact hole 11, there is no trouble in this instance.
However, referring to FIG. 4, when the aluminum wire 14 is to be connected to the transfer gate 5 by way of the contact hole 16, an aluminum-silicon-copper alloy is put into the contact hole 16 by sputtering. This sputtering provides a poor coating property for a sidewall face of the contact hole 16 and hence does not provide such an ideal structure as shown in FIG. 4. This method has such problems as described below with reference to the following figures.
FIGS. 5A to 5D are schedule views of conventional steps in forming a contact hole in a multi-layer film and are represented in sectional views.
Referring to FIG. 5A, a multi-layer film 15 is formed on a substrate 17. The multi-layer film 15 includes a lower film 18, an intermediate film 19 and an upper film 20. The substrate 17 is, for example, a silicon substrate or a wire (polycrystal silicon, an aluminum alloy or the like). The lower film 18 is a layer insulating film, for example, a silicon oxide film. The intermediate film 19 is an extension of a capacitor dielectric film, for example, a silicon nitride film. The upper film 20 is a layer insulating film, for example, a silicon oxide film.
Subsequently, referring to FIG. 5B, a resist film 21 is formed on the multi-layer film 15. After then, the resist film 21 is patterned so that an opening may be formed at a portion of the resist film 21 at which a contact hole is to be formed. After then, using the resist film 21 as a mask, the multi-layer film 15 consisting of the lower film 18, intermediate film 19 and lower film 20 is etched until a surface of the substrate 17 is exposed, thereby forming a contact hole 22. Formation of the contact hole 22 is carried out by etching the upper film 20, intermediate film 19 and lower film 18 at a stroke normally using plasma etching which presents a high anisotropic property. Where the anisotropic property is high, the selectivity in etching between a silicon oxide film (18, 20) and a silicon nitride film (19) is low, which allows a smooth etched face to be obtained. At this stage, a native oxide film 60 is produced at the bottom of the contact hole 22. After then, the resist film 21 is removed.
Then, after removal of the resist film 21, referring to FIG. 5C, the substrate 17 is immersed for a short period of time in dilute solution of hydrofluoric acid or ammonium fluoride to remove a very thin native oxide film 60 of scores angstroms which exists on a surface of the substrate 17. Such removing processing of a native oxide film is a step essential to improvement in ohmic property of a junction between the substrate 17 and an aluminum wire which is formed later. However, since the wet type removing processing of a natural oxide film is chemical etching, the selectivity in etching is high. Consequently, a phenomenon takes place that a silicon oxide film (18, 20) is etched while a silicon nitride film (19) remains not etched, and an eave-like projection 19a of the silicon nitride film (19) is produced on a sidewall face of the contact hole 22 as shown in the figure. If the aluminum wire 14 of an aluminum alloy or the like is formed by sputtering while such an eave-like projection 19a as described above remains present, referring to FIG. 5D, the aluminum wire 14 will not come around to a location below the eave-like projection 19a. As a result, the coating property of the aluminum wire 14 is deteriorated extremely on the sidewall face of the contact hole 22, and in a worst case, the aluminum wire 14 is disconnected at a portion thereof on the intermediate film 19 (silicon nitride film) as shown in the figure, thereby causing an imperfect conducting condition. Or else, even if disconnection does not take place, the thickness of the aluminum wire 14 is reduced at a portion thereof on the eave-like projection 19a, which makes a problem of reliability.
Further, with the conventional method, a problem sometimes takes place also when a contact hole is formed.
FIGS. 6A to 6E are schedule views of a process of forming a contact hole in a multi-layer film using both of wet etching and anisotropic etching.
Referring to FIG. 6A, a multi-layer film 15 is formed on a substrate 17. The multi-layer film 15 consists of a lower film 18, an intermediate film 19 and an upper film 20. The lower film 18 and the upper film 20 are, for example, silicon oxide films. The intermediate film 19 is, for example, a silicon nitride film.
Subsequently, referring to FIG. 6B, a resist film 21 is formed on the multi-layer film 15. Then, the resist film 21 is patterned so that an opening may be formed at a portion of the resist film 21 at which a contact hole is to be formed. Then, using the resist film 21 as a mask, isotropic etching of the multi-layer film 15 is carried out with etching liquid of hydrogen fluoride. The reason why wet etching is additionally used for the formation of a contact hole is that it is intended to shape an upper end portion of the contact hole into a tapered configuration. However, since such a wet etching is chemical etching, the selectivity in etching is high. Consequently, such a phenomenon takes place that a silicon oxide film (18, 20) is etched while a silicon nitride film (19) remains not etched, and an eave-like projection 19a of the silicon nitride film (19) is produced as shown in the figure. Then, referring to FIG. 6C, using the resist film 21 as a mask, anisotropic plasma etching is carried out to form a contact hole 22 which extends to the substrate 17.
After then, referring to FIG. 6D, if the resist film 21 is removed, then the contact hole 22 is obtained wherein the eave-like projection 19a of the silicon nitride film 19 is projected from a sidewall face of the contact hole 22.
If an aluminum wire 14 of an aluminum alloy or the like is formed by sputtering while such an eave-like projection 19a as described above remains present, then the aluminum wire 14 will not come around to a location below the eave-like projection 19a as shown in the figure. As a result, the coating property of the aluminum wire 14 is deteriorated extremely at the sidewall face of the contact hole 2, and in a worst case, the aluminum wire 14 is disconnected at a portion thereof on the silicon nitride film (19) as shown in the figure to cause an imperfect conducting condition. Or else, even if disconnection does not take place, the film thickness of the aluminum wire 14 is reduced at a portion thereof on the eave-like projection 19a, which makes a problem of reliability.
Further, while, in the conventional examples described above, the case is described wherein an intermediate film is projected as an eave-like projection from a wall face of a contact hole, referring to FIG. 7, when a spin on glass film (SOG film) having low density is used for an intermediate film 19, a phenomenon takes place that the intermediate film 19 recedes from a wall face of a contact hole 22 by etching before formation of an aluminum wire (etching for the removal of a native oxide film) so that a groove 19b in the form of a depression is produced on a sidewall of the contact hole 22. If an aluminum wire 14 is formed by sputtering while such a groove 19b in the form of a depression as described above remains present, then disconnection of the aluminum wire 14 takes place as shown in the figure. It is to be noted that, since members or elements denoted by reference numerals 14, 17, 18, 20 and 22 in FIG. 7 are quite the same as those described hereinabove with reference to FIG. 5D, description thereof will not be repeated.
The problem of disconnection of an aluminum wire in a contact hole described above will hereinafter be a progressively serious problem due to an increase of layers in a film arising from promotion of fine structures of semiconductor devices, a deterioration of the coating property of metal wiring films arising from an increase of the aspect ratio of contact holes, and so forth.
In order to solve such problems as described above, Japanese Patent Laying-Open No. 64-57717 discloses a technique of protecting a sidewall face of a contact hole with a polycrystal silicon film.
In particular, referring to FIG. 8, a multi-layer film 15 is formed on a substrate 17. The multi-layer film 15 consists of an upper film 23 and a lower film 24. The multi-layer film 15 has a contact hole 22 formed therein which extends to the substrate 17. A polycrystal silicon film 25 is formed in such a manner as to cover a sidewall face of the contact hole 22. Such a construction as described above prevents formation of such an eave-like projection or a groove in the form of a depression as described above. However, the method has the following problems.
In particular, when the multi-layer film 15 is etched to form the contact hole 22, the surface of the substrate 17 is acted upon by an impact energy upon etching. Further, since the polycrystal silicon 25 is formed on the sidewall face of the contact hole 22, the surface of the substrate 17 is acted upon again by an impact energy upon etching. Since the surface of the substrate 17 is acted upon by an impact energy twice, there is a problem that the substrate 17 is damaged to cause leak current.
Further, there is a problem that there remains a polycrystal silicon film as a residue during etching of the polycrystalline silicon on the bottom of the contact hole 22 and consequently an ohmic junction cannot be obtained between the aluminum wire 14 and the substrate 17.
In addition, according to the method, a polycrystal silicon film is deposited on the substrate 17 using chemical vapor deposition and is then dry etched to form a polycrystal silicon film 25 on the sidewall face of the contact hole 22. Upon such dry etching, fragments of polycrystal silicon are scattered as foreign particles and stick to every portion of the semiconductor device. Accordingly, there is a problem that such polycrystal silicon fragments may make a cause of short-circuiting between wires.